The present invention relates generally to a system for mastering, recording and replicating optical digital media, such as optical digital discs. 2. Description of the Prior Art
Digital Compact Disc technology was developed over twenty years ago by various organizations, including Philips Electronics, Sony, Thomson and Discovision Associates (DVA). This technology (with extensions and improvements) has been adopted as a standard by the largest consumer electronics and computer companies in the world. The relatively large storage capacity and low unit cost of both the discs and the playback units have resulted in worldwide sales and licensing revenues measuring many billions of dollars per year, exclusive of the content of the discs themselves. This technology has become a worldwide standard for permanent digital data storage of all types.
Digital Compact Discs (or "CDs") consist of a disc made of high-grade plastic approximately 120 mm in diameter and 1.2 mm thick coated with a thin (50 nm) layer of aluminum. These discs may contain up to 1.2 billion bytes of digital information. Commonly used error correction schemes typically reduce the effective storage capacity of these discs to approximately 680 million bytes of digital information.
The basic underlying principal of the digital optical recording implemented in Compact Disc technology is the local reflectivity modulation where the pluralities of areas with high and low reflectivity represent individual bits of data. The most common method for local reflectivity modulation is so called "phase pit" method. The phase pits must be exactly as deep as the length of one quarter of the wavelength of the light source reading the data (approximately 120 nm). The phase pits can be replaced by amplitude objects where reflectivity is reduced due to discontinuity in the reflective coating or because of the light scattering on convex or concave microscopic features.
The pits (or the amplitude objects) on CDs are arranged in a spiral pattern beginning about 20 mm from the center of "hub" of the disc and continuing in a single spiral track to within a few mm of the outside edge of the disc. The entire length of this spiral track is considered to be a long line of locations where phase pits may or may not exist. If red laser light bouncing off a location detects a pit, a photodetector and related circuits interpret its presence as the number "1". If the location does not contain a pit, this will be interpreted to be "0" (zero). This continuous string of ones and zeros comprises the digital information recorded on the disc. The nominal width of a pit is 0.6 .mu.m; the distance between one loop of the spiral track and either its inner or outer neighboring loop is 1.6 .mu.m.
The existing industrial process for making CDs can be divided into three separate operations: mastering, stamper production, and replication. The general steps of prior art CD processing are described below.
A. Mastering
1. Data is premastered according to a specific format; PA1 2. An optically polished glass disk (or the glass master) coated with a photoresist layer is provided; PA1 3. The glass master is exposed in a laser beam recorder by a focused laser beam modulated accordingly to the premastered data. The focused laser beam follows a spiral trajectory on the surface of the glass master with the light intensity turned on and off by an acousto-optical modulator. The exposed areas of the photoresist layer correspond to position and dimension of the phase pits; PA1 4. The glass master is developed, and exposed areas of photoresist are washed out; PA1 5. An inspection may be performed after each step described above. PA1 1. A thin layer of silver is deposited over the photoresist pattern on the glass stamper by vacuum evaporation; PA1 2. A thick layer of nickel is deposited over silver the by electroplating, forming a nickel plate father. One father is produced; PA1 3. The nickel father is a negative replica of the glass master (i.e. protrusions correspond to the phase pits). The nickel father can be used as an injection molding stamper, but it is usually not used as a stamper given its high cost to produce. For this reason, several mothers (positive replicas) are produced by electroplating and separation; PA1 4. Stampers (negative copies of the glass master) are produced from the mothers by electroplating and separation; PA1 5. An inspection may be performed after each step described above. PA1 1. The nickel stamper is used in a high pressure injection molding of polycarbonate CD substrates; PA1 2. After cooling the substrate is coated with a reflective layer of aluminum by sputtering; PA1 3. A protective layer is spin-coated atop of the aluminum layer and subsequently cured by UV radiation; PA1 4. The diffraction efficiency of the spiral tracks is used as a final inspection criterion. PA1 The mastering and stamper production require prolonged use of expensive equipment and facilities; PA1 The prior art process is low speed and discrete. Each CD is handled separately; and it takes on at least 4 seconds to fabricate a CD, a relatively lengthy amount of time; PA1 The prior art process involves high temperature and high pressure. The plastic melts at a temperature of approximately 300.degree. C., and is injected with a force 20-40 tons. Because of significant nonuniformly distributed stresses due to the high pressure injection and temperature gradients during rapid cooling, the birefringence problem (i.e. anisotropy of the refraction index) arises; PA1 In order to minimize the birefringence arising from process-induced optical nonuniformities, a very expensive polycarbonate plastic is used as a substrate material; PA1 The synthesis process for the polycarbonate resins of the substrate include a chlorinating step. Residual chlorine atoms attack the reflective aluminum coating of a CD, reducing the CD's expected lifetime; PA1 Due to the complexity and vulnerability of the injection molding process, very high capital investment is required to meet the rapidly growing demand for audio CD's and CD ROM's; PA1 Today, a commercially viable plant can be built for US$75 million; very large facilities can require over US$1 billion. Using ultrapure materials, CDs can be produced with a fully burdened cost of at least approximately US 40 cents per unit.
B. Stamper production
C. Replication
The labeling and packaging steps involved in producing replicas are excluded from the above description of the prior art because they are carried out most commonly off-line. The mastering operation (including photoresist spinning and deposition) typically requires 3-4 hours in a class 100 cleanroom facility. Stamper production requires 5-8 hours in a class 100 cleanroom facility. Finally, an efficient injection molding replication yields on average 1 CD per 4 seconds.
The above-described prior art manufacturing method for CDs has, among others, the following disadvantages:
In order to overcome some of the problems associated with the prior art, alternatives to the above-described prior art methodology have been developed. For example, it has been proposed that mechanical and temperature stress can be reduced where a method of embossing is used instead of injection molding. In this technique, phase pits are replaced by amplitude objects. The reflectivity of the amplitude objects is reduced because of light scattering at the edges as well as due to the discontinuities in the metal coating selectively deposited by the shadow mask method. Because of the complexity of the shadow mask fabricated by the metal evaporation, as well as other disadvantages, the embossing method has not to date become a viable commercial option.
Another alternative for the high temperature/high pressure injection molding process is the contact photolithography replication method suggested by U.S. Pat. No. 4,423,137, assigned to Quixote Corporation, aid generally depicted in FIGS. 1A, 1B, 1C and 1D. As shown in FIG. 1A, this process consists of the use of contact photolithography with a flat rigid master mask, which may comprise a flat glass substrate 1 coated with a layer of reflective metal 2 having apertures 3 corresponding to the pit pattern of a CD. The flat rigid master mask is replicated onto a flat rigid substrate 4 covered by a reflective layer 5 and a photoresist layer 6. In FIG. 1B, the areas 61 of the photoresist 6 are exposed to light, and are removed, exposing the underlying areas 51 of the reflective coating 5. In FIG. 1C, the areas 51 are then etched, and the photoresist 6 is removed in FIG. 1D. The resulting structure represents the plurality of amplitude objects sized and distributed over the surface of the substrate corresponding to openings in the reflective coating on the master plate. In the subsequent steps of applying a protective coating, laminating onto a rigid transparent disk, and labeling, a CD compatible with the ISO 9660 standard may be produced.
Contact photolithography has been well known since the early 1960s as a method for microscopic pattern transfer in semiconductor device fabrication. The main requirement for the successful implementation of contact lithography is to reduce the gap between the photomask (e.g., metal patterned master plate 1 and 2 in FIG. 1A) and the photoresist coating 61 of the substrate 4. This requirement can arguably be met for small surface areas (1-5 cm.sup.2), but it becomes extremely difficult, if not impossible, to reliably control the gap for large flat surface areas--for example a 12 cm in diameter CD substrate. Moreover, if contact lithography is intended to be applied for a high throughput replication process, it becomes impossible to maintain a uniformly small gap between the master and the substrate. For this and other reasons, the replication process described generally in U.S. Pat. No. 4,123,137, though possible in theory, cannot be implemented in commercial practice.
It is generally accepted that a continuous manufacturing process has substantial advantages over a batch or discrete process, as a continuous process is much faster, more reliable and less expensive. Obviously, injection molding techniques of the prior art are essentially discrete methods for CD manufacturing. Thus, the introduction of a continuous CD replication method would constitute a substantial improvement over the prior art.
A publication entitled "Continuous Manufacturing of Thin Cover Sheet Optical Media", written by W. Dennis Slafer et al. of Polaroid Corporation, and published in SPIE Vol. 1663 Optical Data Storage (1992) at page 324 (the "Polaroid article"), discloses a continuous manufacturing method for CDs. In this method, a continuous web of a thin film substrate is embossed by micro protrusions on the surface of a roller, and consequently is metallized to achieve reflectivity, and is laminated onto a thick transparent plastic sheet in order to add the thickness up to the standard value. This composite plastic web is handled and transported with constant speed during the entire replication process until it is separated into individual CDs. The replication method introduced by the Polaroid article utilizes well-known techniques for the continuous web handling, printing, and lamination. However, microembossing of a plastic film by microscopic protrusions on a curved surface of a roller is difficult to control, especially at high velocities of the web. Thus, this prior art technique also has significant disadvantages that make it impractical as an effective method for replicating CDs.
It is worth noting that the existing standard for CD Audio and CD ROM media is currently being replaced by a new standard which provides a greater information storage capacity. The competing new standard such as Digital Video Disc (DVD) utilizes smaller micro-features and smaller track pitch arranged in multilayer structures. With these new requirements, the potential of the prior art injection molding method is approaching its limit in the microscopic spatial resolution as well as for the process yield. Additionally, the new CD technology is making other known prior art replication techniques even more impractical. There is therefore a significant need for an alternative method and system for replicating optical media having high-resolutions and for replicating new media types, such as those having multilayer structures and other new geometries.